Steering columns for motor vehicles have a steering shaft with a steering spindle, at the rear end of which in the direction of travel, which faces the driver, a steering wheel is attached for introducing a steering command by the driver. The steering spindle is mounted in a manner rotatable about its longitudinal axis in an actuating unit, which is held at the vehicle body by a carrying unit. There can be a longitudinal adjustment on account of the fact that the actuating unit is received in a casing unit, connected to the carrying unit, which is also referred to as guide box or box-section swinging fork, in a telescopically displaceable manner in the direction of the longitudinal axis. A height adjustment can be realized by virtue of the actuating unit or a casing unit receiving the latter being pivotably mounted at the carrying unit. Adjusting the actuator unit in the longitudinal or height direction allows an ergonomically comfortable steering wheel position to be set relative to the driver position in an operating position, also referred to as a drive or actuating position, in which there can be a manual steering intervention.
For the purposes of adjusting the actuating unit relative to the carrying unit, the prior art has disclosed the provision of a motor-driven adjustment drive with a drive unit that includes an electrical servomotor which is connected to a spindle drive—as a rule, via a transmission—that includes a threaded spindle screwed into a spindle nut. By way of the drive unit, the threaded spindle and the spindle nut are driveable to rotate against one another about an axis, namely the threaded spindle axis, as a result of which the threaded spindle and the spindle nut can be moved in translational fashion to one another or away from one another, depending on the direction of rotation. In one example, the threaded spindle is driveable to rotate about its threaded spindle axis by the drive unit which is fixedly connected to the actuating unit or the carrying unit and engages in the spindle nut which is fixedly attached in relation to a rotation about the threaded spindle axis at the carrying unit or, alternatively, at the actuating unit. In the direction of the threaded spindle axis, the threaded spindle is supported at the carrying unit or at the actuating unit and the spindle nut is accordingly supported at the actuating unit or, alternatively, at the carrying unit such that a rotational drive of the threaded spindle brings about a translational adjustment of carrying unit and actuating unit relative to one another in the direction of the threaded spindle axis. Therefore, this version is also referred to as a rotational spindle drive.
In an alternative version, the threaded spindle is coupled to the carrying unit or, alternatively, to the actuating unit in a non-rotational manner with respect to rotation about its threaded spindle axis and the spindle nut is rotationally mounted accordingly at the actuating unit or, alternatively, at the carrying unit but it is stationary in the direction of the threaded spindle axis. Like in the first example, the threaded spindle is supported at the carrying unit or at the actuating unit in the direction of the threaded spindle axis and the spindle nut is accordingly supported at the actuating unit or at the carrying unit such that the threaded spindle is displaceable in a translational manner in the direction of the threaded spindle axis by virtue of the spindle nut being driven to rotate by the drive unit. This version is also referred to as a plunger spindle drive.
Like in the first above-mentioned alternative, a translational adjustment of carrying unit and actuating unit relative to one another is brought about in the direction of the threaded spindle axis as a result of the rotational drive of the threaded spindle. In both, the spindle drive forms a motor-driven adjustment drive that is effective between carrying unit and an actuating unit, the adjustment drive facilitating the adjustment of the actuating unit relative to the carrying unit for adjustment purposes.
In order to realize a longitudinal adjustment of the actuating unit in the direction of the longitudinal axis of the steering spindle, the spindle drive of an adjustment drive can be arranged between the actuating unit and a casing unit that receives the latter in an axially longitudinally displaceable manner, the casing unit also being referred to as a guide box or box-section swinging fork and being connected to the carrying unit, wherein the threaded spindle axis can be aligned substantially parallel to the longitudinal axis. For the purposes of height adjustment, a spindle drive can be arranged between the carrying unit and an actuating unit that is mounted thereon so as to be pivotable in height or a casing unit, in which the actuating unit is received. A motor-driven longitudinal and height adjustment can be embodied at a steering column individually or in combination.
The drive of the spindle drive is effected by the drive unit by way of a gear wheel that is driveable to rotate about its axis, which is identical to the threaded spindle axis, the gear wheel being connected to the spindle nut or to the threaded spindle for rotation therewith, depending on the embodiment of the spindle drive. The gear wheel has a toothed portion in the form of a spur gear, with an outer circumferential toothing or worm toothing.
The gear wheel in each case has a circumferential bearing face that is coaxial with the axis on each of its two end sides. In a bearing arrangement, the bearing faces are arranged between two corresponding outer bearing faces that are arranged on the sides of the two outer bearing rings facing one another, between which the gear wheel is rotatably mounted. As seen from the gear wheel, the outer bearing rings are supported and affixed on the outside at the bearing housing in the axial direction. As a result, holding and adjustment forces that act on the gear wheel in both axial directions of the threaded spindle axis on the spindle drive are transmitted via the gear wheel and the outer bearing rings onto the bearing housing, and are supported from there at the actuating unit or the carrying unit.
Such an adjustment drive with a rotatably mounted and axially supported gear wheel is known from U.S. Pat. No. 4,967,618, for example. The bearing faces of the gear wheel have raceways for rolling bodies, specifically ball-bearing raceways of ball bearings. Ball bearings as rolling bodies are arranged between these ball-bearing raceways and these opposing, corresponding ball-bearing raceways in the axially or obliquely opposing outer bearing faces of the outer bearing rings. As a result, a bearing arrangement is formed, in which the gear wheel is mounted between two axial pressure bearings in a manner supported in the axial direction, the pressure bearings in each case being formed by a bearing face, an outer bearing face and the ball bearings arranged therebetween. The outer bearing rings are rigidly affixed to the bearing housing.
During assembly, a bearing arrangement is set by axially positioning and affixing the outer bearing rings relative to one another such that the ball bearings roll without play between the ball-bearing raceways. So that, where possible, no bearing play occurs during operation in order to ensure low-noise running, the outer bearing rings are braced against one another in the bearing housing such that the bearing faces and outer bearing faces are pressed against one another in the axial direction. Here, on the one hand, the axial setting force exerted on the outer bearing rings must be high enough so that play-free running of the ball bearings is ensured, even under temperature variations and in the case of wear. However, on the other hand, the setting force must not be too high, as this could result in an elevated breakaway torque of the gear wheel and increased wear.
Thus, a need exits for an improved adjustment drive and a steering column with an improved adjustment drive for a motor vehicle, which has greater running smoothness and less wear.